After the Industrial Revolution, humans established modern society while consuming huge amounts of fossil fuels which increases carbon dioxide concentration in the air, and the increase of the carbon dioxide concentration is further promoted by environmental destruction such as deforestation or the like. Since global warming is caused by an increase in greenhouse gases such as carbon dioxide, Freon, or methane in the air, it is important to decrease the concentration of carbon dioxide that significantly contributes to global warming, and various researches into emission regulations or stabilization of carbon dioxide have been conducted around the world.
Among them, a copolymerization reaction of carbon dioxide and an epoxide found by Inoue et al. has been expected to be a reaction capable of solving the global warming problem, and research has been actively conducted in view of using carbon dioxide as a carbon source as well as in view of chemical fixation of carbon dioxide. Particularly, a poly(alkylene carbonate) resin formed by polymerization of carbon dioxide and an epoxide has been recently spotlighted as a kind of biodegradable resin.
Various catalysts for preparing this poly(alkylene carbonate) resin have been studied and suggested in the past, and as a representative catalyst, a zinc dicarboxylate-based catalyst such as a zinc glutarate catalyst in which zinc and dicarboxylic acid are bonded to each other has been known.
The zinc dicarboxylate-based catalyst as described above, represented by the zinc glutarate catalyst, is formed by reacting a zinc precursor and a dicarboxylic acid such as glutaric acid with each other, and has a fine crystalline particle shape. However, it was difficult to control the zinc dicarboxylate-based catalyst having the crystalline particle shape to have a uniform and fine particle size during a preparation process thereof. The existing zinc dicarboxylate-based catalyst has a particle size of a nanometer scale, but an aggregate having an increased particle size and a decreased surface area is formed in a medium by aggregation of catalyst particles such that the activity may be deteriorated upon preparing the poly(alkylene carbonate) resin.
In this regard, it is known that as the zinc precursor used in the preparation of the zinc dicarboxylate-based catalyst has a smaller size, the produced catalyst has higher activity.
In general, zinc oxide powder used as a zinc source (zinc precursor) in the preparation of the zinc dicarboxylate-based catalyst is an ionic compound having a particle size of a few tens of nanometers to several hundred micrometers and a specific surface area of about 10 m2/g. This zinc oxide powder is highly polar, and thus is dispersed well in polar solvents, but particles are aggregated with each other in polar solvents to form very large aggregates. When a catalytic synthetic reaction is allowed under the aggregation of particles, a non-uniform reaction occurs, and therefore heterogeneity in crystallinity of the resulting catalyst is increased to decrease activity of the catalyst, in view of the entire reaction system.
In this regard, International Patent Publication No. WO 2013/034489 and U.S. Pat. No. 7,405,265 disclose a method of increasing specific surface area of the zinc dicarboxylate-based catalyst by using an emulsifier or a non-ionic surfactant as a templating agent in a polar solution. However, the results of the previous literatures or studies showed that when zinc oxide and glutaric acid are used as a synthetic material of the zinc dicarboxylate-based catalyst, a catalyst synthesized in a non-polar solvent generally has higher activity than a catalyst synthesized in a polar solvent. Thus, there is still a limitation in the improvement of the catalytic activity by the method of using the polar solvent.
Further, International Patent Publication No. WO 2011/107577 discloses a method of synthesizing a catalyst after increasing a specific surface area of a zinc source used in the preparation of an organic zinc catalyst by surface treatment of the zinc source with organosilane. However, this method requires several steps (reaction, selection, drying, etc.) in the surface treatment of the zinc source with organosilane, and therefore there is a limitation in that this method is inefficient considering an improvement degree of the catalytic activity.
For this reaction, there is often a case that the existing known zinc dicarboxylate-based catalyst has a relatively large particle size and a non-uniform particle shape. However, when the zinc dicarboxylate-based catalyst having the above shape is applied to a polymerization process for preparing a poly(alkylene carbonate) resin, a sufficient contact area between a reactant and the catalyst is not secured, and therefore sufficient polymerization activity may not be exhibited. In addition, there is often a case that the activity of the existing zinc dicarboxylate-based catalyst itself is not satisfactory. Further, in the case of the zinc dicarboxylate-based catalyst, it is not easy to disperse and control the catalyst particles in a reaction solution due to non-uniformity of the particle size.